1. Field of the Invention
The invention relates to modifying agents for melamine-formaldehyde resins, a highly reactive, modified thermosetting resin based on a melamine-formaldehyde condensation product and the use of the modifying agent for the preparation of thermosetting resins, laminates, and coated sheets.
2. Relevance of the Prior Art
Precondensed, aqueous or aqueous-alcoholic solutions of melamine-formaldehyde resins are known in the art to be outstandingly suitable for impregnation of absorbent carrier materials, such as, for example, paper webs or non-woven materials or fabrics, which are dried after the impregnation and finally processed to laminates (layered material) by hot pressing, or for improving the surface of derived timber products, for example hardboard or wood chipboard. Suitable resins are taught, for example, in U.S. Pat. No. 2,197,357 to Widmer et al issued Apr. 16, 1940.
Hard, scratch-resistant surface coatings which are resistant to water and chemicals are obtained using such melamine impregnating resins. Such properties are required, above all, in furniture construction.
The use of pure melamine resin for impregnating carrier materials meets with certain difficulties inasmuch as the pressing pressures must be very high and the pressing times must be relatively long. Resin films finished with such resins and pressed, for example, onto chipboard are moreover susceptible to cracking, especially after a relatively long storage time, which is why improvements have been sought in this respect. Possibilities for avoiding such difficulties are known in the art and resins suitable for particularly rapid curing and processing, known as short cycle resins, have been developed. These advancements may include the modification of melamine resins by replacing some of the melamine by other condensable products, such as urea or glycols. For example, GB 1 464 014 claims an impregnating resin based on an aqueous condensation product of melamine and formaldehyde that is characterized by the melamine replacing addition of saccharose or .alpha.-methylglycoside and 0.5 up to 7% by weight of the resin part in the solution of a diol with 2 to 4 carbon atoms, such as ethylenglycol, 1,3- and 1,2-propylenglycol or 1,2-, 1,3- or 1,4-butandiol.
Such modified resins have improved flow properties, but although useful surfaces have already been obtained with them under lower pressure and over shorter reaction times, their properties were not satisfactory in all respects. In particular, a more pronounced tendency to yellow at higher temperatures which are recommended if short reaction times are applied and a reduced gloss of the surfaces produced with such resins was apparent.